Teach MemoryBuiltins and InstructionSimplify that operator new never returns NULL.

This is safe per C++11 18.6.1.1p3: [operator new returns] a non-null pointer to
suitably aligned storage (3.7.4), or else throw a bad_alloc exception. This
requirement is binding on a replacement version of this function.

Brings us a tiny bit closer to eliminating more vector push_backs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191310 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstSimplify/call.ll

@@ -101,3 +101,23 @@ define float @test_idempotence(float %a) {
 
   ret float %r4
 }
+
+define i8* @operator_new() {
+entry:
+  %call = tail call noalias i8* @_Znwm(i64 8)
+  %cmp = icmp eq i8* %call, null
+  br i1 %cmp, label %cast.end, label %cast.notnull
+
+cast.notnull:                                     ; preds = %entry
+  %add.ptr = getelementptr inbounds i8* %call, i64 4
+  br label %cast.end
+
+cast.end:                                         ; preds = %cast.notnull, %entry
+  %cast.result = phi i8* [ %add.ptr, %cast.notnull ], [ null, %entry ]
+  ret i8* %cast.result
+
+; CHECK-LABEL: @operator_new
+; CHECK: br i1 false, label %cast.end, label %cast.notnull
+}
+
+declare noalias i8* @_Znwm(i64)